1. Field of the Invention
The present invention relates to an optical module for use in a high-speed optical communication apparatus, a measuring instrument, etc.
2. Description of the Related Art
An optical module such as a light emitting module or a photodetecting module for use in an optical communication apparatus or the like is configured by accommodating an optical element such as a laser diode or a photodiode and other components in a metal cylindrical package called a coaxial type package which is low in cost and easy to handle, and connecting the optical element and the other components through wires or the like to lead terminals for supplying a signal and power. In driving the optical element accommodated in the coaxial type package, band degradation or the like is caused by the influence of a stray capacitance, stray inductance, etc. present in the package structure and the internal components, so that there arises a problem in characteristics.
More specifically, the lead terminals for connecting the optical element in the package to an external component have a function as an inductor and also have a capacitance to the metal package. Accordingly, in the case of using this optical module in a GHz band, resonance occurs at a certain frequency and the band becomes narrow, so that desired characteristics cannot be obtained. As a method for solving such a problem at present, it is known that a capacitive component such as a capacitor is connected near the package between a lead terminal mechanically and electrically connected to the package and another lead terminal electrically connected to the optical element in the package and having a potential different from the potential of the package, thereby short-circuiting a stray capacitance. As another method, it is known that the lead terminals are cut into minimum lengths and a capacitive component (e.g., capacitor) is located very close to a wiring board to which the lead terminals are connected.
In such conventional optical modules, the connection of a capacitive component between the lead terminals near the package has a disadvantage in mountability and reliability, and there is a limit to the location of the capacitive component between the lead terminals near the package because of the limitation to bending of another functional terminal. Thus, the above conventional methods have little effect on a reduction in stray capacitance and stray inductance of the high-frequency component accommodated in the package. In the case of locating a flexible printed wiring board near the package and mounting components on the flexible printed wiring board, a high-speed signal lead terminal is also connected to the flexible printed wiring board, so that impedance matching cannot be effected because of the limitation to the material (permittivity) of a flexible member of this printed wiring board and the thickness of this printed wiring board, resulting in a problem such that a desired optical waveform cannot be obtained.
In the case of a combined transmission/reception optical element package for an access system such as fiber to the home (FTTH), the ground potential of the package becomes different from that of a main printed wiring board with only a grounding lead terminal for each optical element, causing a characteristics degradation due to power supply noise. Further, with only a power supply lead terminal, the impedance becomes high to cause the radiation of radio waves, resulting in a problem such that EMI characteristics cannot be ensured.
Some conventional structures of such an optical module will now be described with reference to the drawings. Referring to FIG. 1, there is shown a plan view of an optical module 2A as a first example of the prior art. FIG. 2 is a cross section taken along the line 2-2 in FIG. 1, and FIG. 3 is a cross section taken along the line 3-3 in FIG. 1. The optical module 2A includes an optical element package 4. The optical element package 4 includes a metal casing and an optical element accommodated in the metal casing. The optical element is a light emitting element such as a laser diode or a photodetecting element such as a photodiode. In the case that the optical element is a light emitting element, a drive circuit for driving the light emitting element is mounted on a printed wiring board 6. In the case that the optical element is a photodetecting element, a preamplifier is accommodated in the metal casing and a post amplifier is mounted on the printed wiring board 6.
The optical element package 4 has a power supply lead terminal 10 and a grounding lead terminal 12. These lead terminals 10 and 12 are cut into suitable lengths and connected to the printed wiring board 6 by soldering or the like. A capacitor 16 for reducing a stray capacitance is mounted on the printed wiring board 6 so as to connect the power supply lead terminal 10 and the grounding lead terminal 12. A high-speed signal lead terminal 8 is also cut into a suitable length and connected to the printed wiring board 6. Reference numeral 14 denotes a lead terminal connected to a monitoring photodiode accommodated in the optical element package 4. In the optical module 2A, the distance between the optical element package 4 and the printed wiring board 6 must be ensured to some extent because of the limitation to bending of each lead terminal, and the position of the capacitor 16 is therefore far from the optical element package 4, so that a reduction in stray capacitance cannot be so expected.
Referring to FIG. 4, there is shown a plan view of an optical module 2B as a second example of the prior art. FIG. 5 is a cross section taken along the line 5-5 in FIG. 4, and FIG. 6 is a cross section taken along the line 6-6 in FIG. 4. The optical module 2B is different from the optical module 2A shown in FIG. 1 in the point that the capacitor 16 is directly mounted between the power supply lead terminal 10 and the grounding lead terminal 12 adjacent to each other. In this case, the capacitor 16 is mounted to a lead portion having a low working accuracy, so that the mountability is reduced. Furthermore, a stress due to expansion and contraction of each member according to temperature changes is concentrated at a soldered portion, so that there is a possibility of reduction in reliability due to the rupture of the soldered portion.
Referring to FIG. 7, there is shown a plan view of an optical module 2C as a third example of the prior art. FIG. 8 is a cross section taken along the line 8-8 in FIG. 7, and FIG. 9 is a cross section taken along the line 9-9 in FIG. 7. The optical module 2C includes an auxiliary printed wiring board 18 arranged substantially perpendicularly to the main printed wiring board 6 and a flexible printed wiring board 20 for connecting the auxiliary printed wiring board 18 to the main printed wiring board 6.
All the lead terminals 8, 10, 12, and 14 are cut into suitable shorter lengths and connected to the auxiliary printed wiring board 18 by soldering or the like. The capacitor 16 is mounted on the auxiliary printed wiring board 18 at a position near lead terminal connected portions. In this case, a high-speed signal line extends through the auxiliary printed wiring board 18 and the flexible printed wiring board 20, so that the wiring length of the high-speed signal line becomes large to cause an increase in inductance, resulting in characteristics degradation. Further, impedance control is difficult because of the material characteristics of the flexible printed wiring board, causing characteristics degradation.